Up to the present time, in the field of false twist texturing of thermoplastic multi-filament yarns, to improve productivity, developments of new manufacturing technologies as described below have been promoted in the concerned industrial field.
(1) To attempt to promote labor-saving by making packages of an original yarn for false twist texturing and a textured yarn thereof larger and promoting automation of handling of the package.
(2) To attempt to promote energy-saving by developing a heating method, a cooling method etc., with high efficiency.
(3) To attempt to promote space-saving by developing a simple and compact installation.
Furthermore, to achieve an ultrahigh processing at 1,000 m/min or higher has been recently tried. Means used to perform an ultrahigh false twist texturing up to the present time are as follows.
(1) A highly-oriented undrawn yarn (POY; Partially Oriented Yarns) is used as a fed original yarn.
(2) A heater is made longer in accordance with the heater processing speed. In this case, the heater means a so-called dry hot-plate generally heating a metal plate with a smooth surface with which a yarn is brought into contact by using a heating medium such as "Dowtherm".
(3) A yarn-contacting plate made of a metal is used as a cooling device and in addition, a so-called cooling plate positively cooling said yarn-contacting plate with chilled water is used.
(4) A friction-type one capable of twisting with high speed is used as the false twister.
(5) To suppress ballooning of a yarn, a part where the yarn is freely running is made as low as possible.
(6) Polishing of yarn path parts such as guides where the yarns are brought into contact with is made better.
However, when a yarn speed reaches 1,000 m/min or larger using such means as these, the lengths of the dry hot-plate and the cooling plate which have been conventionally used are too short to make sufficient heating-cooling and the quality level of the conventional textured yarn can not be kept. In addition, if the dry hot-plate and the cooling plate are made longer to perform sufficient heating, ballooning is generated on a twisted yarn and if the length is furthermore made longer, it becomes impossible to set the yarn on the devices. In addition, if the yarn speed is increased, ballooning is generated on the twisted yarn, which becomes easily separated from the dry hot-plate and the cooling plate resulting in insufficient heating-cooling. If the texturing speed is increased, the generated ballooning increases yarn breakage, by which it becomes impossible to keep stable production. If the productivity is unstable, it is not possible to reduce cost based on speeding up the process.
Therefore, to keep processing stability at high speed, suppressing ballooning is tried by making the number of twists smaller than that for a texturing condition at low speed, or increasing processing tension, especially twisting tension. But in such a case, it is of course impossible to obtain good physical properties of a textured yarn.
Taking problems of high speed false twisting described above into consideration, the present inventors have extensively studied them and reached the present invention.